The objective of this research is to grow and characterize single crystals of calcium hydroxyapatite. Emphasis will be placed on the hydrothermal growth method since conditions for growth of crystals as large as 7 x 3 x 3 mm have been determined. Variations in growth temperature, temperature gradient and pH will be tried to enhance the rate of growth, to permit an increase in crystal size, and to eliminate mimetic twinning in these monoclinic crystals. Other methods of crystal growth will also be employed. Crystals grown under these various conditions will be characterized according to stoichiometry, chemical composition, crystal structure, and defect nature. Studies will be made of the degree of nonstoichiometry obtainable in hydroxyapatite. Present methods employed for growth are yielding stoichiometric crystals; the introduction of nonstoichiometry by special treatments of crystals (e.g. heating in vacuum) will be investigated. Crystals will also be grown doped with impurity ions selected for their biological significance; these crystals will also be thoroughly characterized. Determination of the point defect character of the various specimens will be accomplished principally by ESR experiments (possibly also ENDOR and optical absorption experiments). This research project will determine how to obtain hydroxyapatite crystals of excellent quality for fundamental studies both here and in other laboratories, studies which can explain the basic properties of this important biological material.